Snow Leopard incompatible with D-Link Modem?

[#1]

I have an Apple iMac, my ISP at home is PlusNet and I am connected to the internet via an ethernet cable and a D-Link ADSL2+ Ethernet modem, model number DSL-320B (which was, incidentally, supplied to me by PlusNet in August 2008).
My operating system used to be Mac OS 10.5 and I had no problems connecting and staying connected to the internet. Recently, however, I purchased Mac OS 10.6 ("Snow Leopard") on DVD and installed it onto my computer. Ever since then my internet connection is intermittent; it seems to stay connected for less than a minute at a time and to be disconnected more often than connected.
I have tried connecting a PC to the internet using my ethernet cable and modem at my home; it had no problems connecting and staying connected.
I have also tried connecting my iMac to the internet at my workplace using their network instead of my modem; it also had no problems connecting and staying connected.
The iMac is not under warranty but the Snow Leopard disk is and I have spoken with "AppleCare" repeatedly and at length. They have talked me through numerous tests and suggested that the modem is probably the source of the problem and that I should replace it.
I have also spoken repeatedly with PlusNet. They say that the connection between my modem and them is working perfectly. They have offered to supply me with a wireless router to replace my modem, provided I pay £6.99 for postage and packing; if I stay with them for 12 months there will be no charge for the router itself. Specifically, they are offering me a choice between a 4 port Thomson Gateway TG 585 version 7 or a 1 port Thomson Speed-touch ST 536. I have never used a wireless router or network.
I am concerned that, if I get one of these routers, it might work no better than my current modem.
I would be particularly interested to hear from anyone else who might have experienced a similar problem and from anyone who has installed Snow Leopard and successfully connects to the internet using one of the Thomson routers mentioned.

[#2]

I haven't had any such issue with Snow Leopard, on either my Mac Mini or MacBook Pro.
I had a similiar-ish issue when I first got my MBP and tried using ethernet, but disabling and enabling the ethernet device sorted that out.
I have used the Thomson 4 port WiFI router at someone else's house with success, but I don't have one at home.

[#3]

It could be that your upgrade has caused an issue on your network card, the best way to test is to try another router in place and if the problem continues then it's likely that it's your Mac at fault.
If you know someone else that is willing to lend you a router for 30 minutes that may be one way to test, otherwise as you've been advised we could send you a router.

[#4]

From what I can see you're not the only one with problems
Take a look at :-
http://www.mac-forums.com/forums/os-x-operating-system/164649-snow-leopard-keeps-dropping-dns.html
http://macosx.com/forums/mac-os-x-system-mac-software/310392-snow-leopard-networking-startup-problem...
Or try a google for 'mac snow leopard router problem'
Hope that helps

[#5]

Thanks for your replies, everyone. I have tried some fixes suggested by others elsewhere but they haven't succeeded. Plus Net are sending me a new 4 port wireless router. (I haven't had to pay a charge for postage after all.)
I would also be interested in hearing from anyone who has installed Snow Leopard and successfully connected to the internet using a D-Link modem such as my current one.

[#6]

Hi tsphillips
My router is a D Link 2640B as supplied by Plusnet so different model from yours but no problems with Snow Leopard

[#7]

Thanks thacky.
I have received and set up my new Thomson Gateway 4-port router and my internet connection now works fine.
I infer that Snow Leopard is incompatible with my D-Link modem.